This invention relates to devices for applying torque very accurately and for aiding in limiting the torque applied to a shaft such as the shaft of a tap, bolts, etc. The limit is typically set at a value above which, the shaft can be expected to break. The invention is particularly related to tapping operations involving very small taps, 2-56 (threads per inch) and finer
Cutting threads in holes in manufacturing operations can be a critical operation. Tap breakage occurs occasionally because the operation requires applying a relatively large torque to a tap that must have a cross section that is limited by the size of hole being tapped. Tapping a number of holes larger than for a 2-56 thread is routinely performed in CNC operations where the tap is inserted into the xe2x80x9cslipxe2x80x9dchuck of a tapping head mounted on a mill. A number of disclosures have appeared which adopt this approach.
For example, U.S. Pat. No. 5,174,424, to Eriksson is for a tapping head of the type having a clutch between the driver and the driven members of the machine which is adjustable to prevent application of excessive torque as described above.
U. S. Pat. No. 5,076,740 to Petrie discloses an axially telescopic tapper including a fixed part adapted to be held in a machine spindle and a movable part having limited axial displacement.
U. S. Pat. No. 4,820,087 to Ikemoto is for a tap drive unit to be installed in a mill spindle and includes a drive member attached to the main spindle of a mill and a guide member with a tap holder screwed into one end and clutching connection to the driver at the other end.
All of these approaches require that the operator operate the mill to position the tap accurately in line with the hole and this is a time consuming procedure. Furthermore, the disclosed chucks are characterized by complex construction that is expensive and complicated compared to the present invention.
The slip chucks, well known in the art, comprise one end attachable to the shaft of a tap and an opposite end detachably engageable to drive such as the spindle of a mill. An adjustment sleeve between the ends of the chuck is positioned by the operator to adjust the strength of coupling between the ends of the slip chuck to an optimum value selected to prevent breakage of the tap.
Before the tapping operation, the xe2x80x9ccouplingxe2x80x9d strength is adjusted to the optimum value by:
4. coupling the coupler end of a calibrated torque wrench to one end of the slip chuck; (The other end of the slip chuck is fixed during the calibration operation.)
5. turning the wrench to the required optimum value of torque as indicated by a dial on the torque wrench;
6. positioning the adjustment sleeve to where the chuck xe2x80x9cslipsxe2x80x9d when the torque exceeds the optimum value
The slip chuck is now prepared for attaching the tap to one end of the slip chuck and for mounting the other end of the slip chuck in the mill.
The torque wrench of the prior art includes a spring enclosed in a housing. One end of the spring is fixed to the end of the housing. The other end of the spring is secured to a coupler that is rotatably mounted in the end of the housing and engages one end of the slip chuck.
A dial plate is mounted on the housing arranged to indicate the angular displacement (hence torque) between one end of the spring secured to the housing and the opposite end of the spring secured to the coupler. Hence, by engaging the coupler to the slip chuck and then turning the housing on its axis, the magnitude of applied torque is indicated by the displacement (rotation ) of the dial plate relative to the coupler. The adjustment sleeve on the slip chuck is positioned to where the slip chuck will release when the applied torque on the wrench approaches the breaking torque of a tap to be used with the slip chuck.
The problem with using the torque metered wrenches presently available to adjust the torque slip value of a slip chuck for preventing breakage of taps 2-56 and smaller is that the torque to be measured with state of the art torque wrenches is within the range of error of reading the required torque.
The range of error is directly related to the repeatability of the dial reading.
When a tap breaks during a CNC operation on parts that requires numerous tapped holes, the numerous untapped holes is not discovered until the tapping operation is complete. Resetting the part in the mill and aligning the already drilled holes with a new tap is a prohibitive operation. Therefore, present practice for tapping holes for 2-56 threads and smaller is to tap the holes by hand. Hand tapping is labor intensive, and also is characterized as being xe2x80x9criskyxe2x80x9d in terms of the experience and care required by an operator to avoid breakage of the small tap.
It is an object of this invention to provide a torque wrench for use in a large variety of applications such as meauring the breaking torque of bolts, setting the xe2x80x9creleasexe2x80x9d torque on slip chucks, etc.
It is another object of the invention that the torque wrench be adaptable for measuring a wide range of torques where the accuracy of mesurement is consistent with the range of torque to be measured. This range can include, for example, 0-80 taps and 1-8 taps. In this regard, the method of use for setting the xe2x80x9cslip value of torquexe2x80x9d of a slip chuck is convenient and reliable in terms of the accuracy and repeatibility required to apply torque to a tap that is size 2-56 and smaller.
Important features of the invention are based on the observation that the origin of the dial scale fails to return to its imitial (zero) position after the torque applied during calibration is removed I have discovered that this failure resides in two sources. One source is memory of flexure (permanent strain) of the spring which is part of the torque wrench and is flexed during the operation. A second source is frictional force between the spring housing and the rotatably mounted coupler on one end of the spring housing when force is exerted to turn the spring housing.. The coupler couples one end of the spring to the end of a shaft such as the shaft of a slip chuck.
This invention is directed toward a torque wrench which includes an elongated housing enclosing an elongated spring. The spring comprises a stack of relatively thin metal strips. One end of the stack of strips is internally secured to one end of the housing. The other end of the strips is secured to a coupler, journalled by bearings to the inside opposite end of the housing. The coupler is attachable to a slip chuck. A dial plate is rotationally mounted on the coupler and may be rotated to where a xe2x80x9ccatchxe2x80x9d (mark) on the housing coincides with the origin on the dial scale when no torque is applied between coupler and housing.
An important feature of the invention is that the force applied to turn the housing and the point of attachment of the end of the stack of springs to the coupler are substatially coplanar with the bearing. This avoids extraneous geheration of torque that is not coincident with the axis of the bearing.
The relative position between dial plate and coupler is maintained by a friction force between the dial plate and coupler. The friction force is exerted by a bellville spring that urges the dial plate against the coupler. Therefore, when there is no torque exerted to rotate the coupler relative to the housing, the origin of the dial scale is positionable to coincide with the xe2x80x9ccatchxe2x80x9d on the housing.
A ring with a cog is mounted free to rotate in a circular slot in the dial plate concentric with the coupler.
When no torque is applied between coupler and housing, the dial plate is rotated to where the origin of the dial scale coincides with the catch on the housing. The ring with cog is rotated to where the cog is against the catch. When torque is applied between housing and coupler, the catch rotates away from the origin of the dial plate and also pushes the cog away from the origin (rotates the ring with cog) to a position adjacent a torque reading on the dial scale. When the torque is removed, the housing rotates to where the catch on the housing returns to coincidence with the origin of the dial scale. The cog on the ring remains in a position relative to the dial scale indicating the applied torque. Therefore the torque reading remains even after the torque has been reduced to zero.
After the torque has been removed so that the catch returns to the origin of the dial scale, then the operator rotates the ring to where the cog on the ring coincides with the catch and origin of the dial scale in preparation for the next reading.
To calibrate a slip chuck so that applied torque between ends of the slip chuck will not exceed an optimum value corresponding to breaking strength of a tap to be attached to the slip chuck, one end of the slip chuck is attached to the coupler and the other end of the slip chuck is fixed. The operator turns the housing so that the optimum value of torque (corresponding to the breaking strength of the tap) is applied to the end of the slip chuck. The adjustment sleeve on the chuck is adjusted to where the slip chuck barely slips. The slip chuck is then detached from the torque wrench and mounted in the drive (mill, or hand wrench) for tapping holes.
An important novel feature of the invention is that the spring comprises a stack of thin metal strips that exhibits no permanent distortion (flexure) when returned to starting position from a twisted position assumed during a calibration operation.
Another important feature is that the bearing support between the coupler and inside housing eliminates drag that would otherwise prevent complete return of the housing to its initial position.
The result of these features is that the catch on the housing will return to its initial position coincident with the origin of the dial scale when torque is removed.
Another important feature is the friction between the dial plate and coupler which is adjustable by virtue of the bellville spring. The friction is adjusted to where the rotational position of the dial plate is stabilized but is not so great as to prevent rotation of the dial plate relative to the coupler when required.